Generally, in the majority of assembly applications (especially those related to the manufacture of disc driver assemblies for computers), the reliability of the joint is dependent upon how accurately and smoothly, the particular fastener or series of fasteners, can be made to clamp the assembly elements together. The clamping force is regulated by the amount of torque applied to the fasteners by production personnel (called torque control).
Engineers establish the theoretical torque value for a particular fastener using formulas that relate tensile stress of the fastener to equivalent stress of the fastener thread that takes into account the material used, thread pitch and minor diameters.
Then, the production personnel take the engineered torque value and after presetting their production tools to provide such torques during the particular mechanical jointing operations, manufacture the assemblies in a rapid manner in which the assembly elements pass from station to station using different preset production tools. Most production lines use production tools in which the torque value for a particular application has been preset for use at the particular station.
In disc drive assemblies, designers require greater storage capacity within a given physical space. Result: the longitudinal spacing between the magnetic discs along the central shaft, becomes less and less. Hence, as torque tolerances for assembly of the disc drives increase, say to tolerances of -+1.5 percent accuracy, conventional mechanical preset production tools have been found to be lacking in certain regards.
For example, where such tools rely upon ratchet type or other means of stepped loading of the fastener, they have been found to create shock waves at the fastener. Such waves are believed to originate at the fastener and are transmitted to the elements to be joined as stepped loading occurs. Where the elements to be assembled are stacks of magnetic discs of a disc drive assembly of a computer, such waves can cause the discs to change position, touch and otherwise inappropriately affect their operations.
One such step leading mechanical tool is shown in U.S. Pat. No. 4,063,474 wherein the described tool loading of the fastener uses a first annular clutch plate that is spring biased (i) longitudinally into contact with a second clutch plate integrally formed within a coextensive cylindrical handle (through a series of longitudinally restrained balls sitting in sets of pairs of longitudinally offset pockets in the clutch plates), and (ii) radially through a second series of bails locked in a series of radial slots in the bit cylinder. The handle, clutch plates (and the bit cylinder) rotate together with the rotation of the handle until the torque at the fastener (through the bit cylinder) is greater than friction response (as provided by the spring) between the second series of balls and the outer surface of the bit cylinder. Then, the bit cylinder remains stationary, with both (or one) of the clutch plates and handle then rotating relative to the axis of symmetry of the bit cylinder. While such tools may be initially accurate, that fact that the clutch plates contact at separate lands associated with the restrained balls about the circumferential extending faces, has been found to contribute to the rise in inaccuracy of the tools with time. It is believed that each radially separated ball and associated pair of pockets as well as the longitudinal end loading of the series of balls against the bit cylinder, non-uniformly contributes to the total force preset into the tool. Moreover, the machining requirements to create the balls and restraining slots and pockets in the bit cylinder and clutch plates, respectively, are usually beyond the capability of production personnel to repair. As a result, repair cannot occur at the work site.
I am also aware of a digitally programmable mechanical electrical production tool in which electrical current is used to accurately drive a servo motor whereby the load can be linearly varied with time, which results in the achievement of accurate torque settings on a linear loading basis. However, due to the high initial cost, experience shows there is still a need for a completely mechanical torque limiting production tool, that is low cost, and that uses an easily serviceable clutch mechanism that (especially for use in micro-torque applications) limits shock wave generation and has high repeatability over many cycles of operations.